This invention relates generally to turbine engines, and specifically to turboprop and turbofan engines for aircraft. In particular, the invention concerns a, variable cycle turbine engine operating in mixed turboprop and turbofan modes.
Gas turbine engines provide efficient, reliable power for a wide range of applications, including aviation, industrial power generation, and commercial heating and cooling. Turbine engines (or combustion turbines) are built around a power core made up of a compressor, combustor and turbine, arranged in flow series with an upstream inlet and downstream exhaust. The compressor compresses air from the inlet, which is mixed which fuel in the combustor and ignited to generate hot combustion gas. The turbine extracts energy from the expanding combustion gas, and drives the compressor via a common shaft. Energy is delivered in the form of rotational energy in the shaft, reactive thrust from the exhaust, or both.
Large-scale gas turbine engines typically include a number of different compressor and turbine sections, which are arranged into coaxially nested spools. The spools operate at different pressures and temperatures, and rotate at different speeds.
Individual compressor and turbine sections are subdivided into a number of stages, which are formed of alternating rows of rotor blade and stator vane airfoils. The airfoils are shaped to turn, accelerate and compress the working fluid, and to generate lift for conversion to rotational energy in the turbine.
Aviation applications include turbojet, turbofan, turboprop and turboshaft engines. Turbojet engines are an older design, in which thrust is generated primarily from the exhaust. Modern fixed-wing aircraft typically employ turbofan and turboprop configurations, in which the low spool is coupled to a propulsion fan or propeller. Turboshaft engines are used on rotary wing aircraft, including helicopters.
Turboprop and turboshaft engines usually include reduction gearboxes to reduce blade tip speeds. The reduction ratio is generally higher for turboshaft engines, due to the larger size of the rotor. Advanced turbofan engines may also include geared drive mechanisms, providing independent fan speed control for reduced engine noise and increased operating efficiency.
Commercial and general-purpose military aircraft are typically powered by two- or three-spool turboprop and turbofan engines, in which the low spool is coupled to a propeller or propulsion fan. Turboprop engines typically employ open-rotor propeller blades, but ducted propellers and unducted turbofans are also known.
Turbofan engines are commonly divided into high and low bypass designs. High-bypass turbofans most of their thrust from the fan, which drives airflow through a bypass duct oriented around the engine core. Low-bypass turbofans generate more power from the core flow, providing greater specific thrust, but at some cost in noise and fuel efficiency. Low-bypass turbofans are used on supersonic jets and other high-performance aircraft
In general, aircraft engine performance depends on precise control of the working fluid flow, and on the relative loading of the different spools. Turboprop and turbofan engines are subject to similar design considerations, but they operate at different airspeeds.
Open-rotor turboprops and unducted turbofans are also subject to substantial noise effects, as compared to turbofan engines, because the blade tips operate at supersonic speeds and there is no nacelle or fan duct to absorb sound. These issues are particularly relevant at higher speeds, where shock wave formation and loss effects are more pronounced, making it difficult to balance the relative contributions of turboprop and turbofan operations.